A so-called Kaminsky catalyst well known as uniform catalyst for olefin polymerization exhibits a high polymerization activity and thus allows the preparation of a polymer having a sharp molecular weight distribution.
As transition metal compounds for use in the preparation of an isotactic polyolefin in the presence of a Kaminsky catalyst there are known ethylenebis(indenyl)zirconium dichloride and ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (as in JP-A-61-130314 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). However, the preparation of a polyolefin in the presence of such a catalyst is normally disadvantageous in that the resulting polyolefin has a small molecular weight and, if low temperature polymerization is effected to obtain a polymer having an increased molecular weight, the catalyst exhibits a reduced polymerization activity.
Further, for the purpose of preparing a high molecular polyolefin, a method has been proposed involving the use of a hafnium compound instead of the foregoing zirconium compound (Journal of Molecular Catalysis, 56 (1989), pp. 237-247). However, this proposed method is disadvantageous in that the catalyst used exhibits a low polymerization activity.
Moreover, dimethylsilylene bis-substituted cyclopentadienyl zirconium dichloride has been proposed (as in JP-A-1-301704, Polymer Preprints, Japan 39 (1990), pp. 1,614-1,616, JP-A-3-12406). Dimethylsilylene bis(indenyl) zirconium dichloride has been proposed (as in JP-A-63-295007, JP-A-1-275609). The use of these compounds allows the preparation of a polymer having a high steric regularity and a high melting point in a relatively low temperature polymerization process but provides a polymer having a low steric regularity, melting point and molecular weight under high temperature polymerization conditions which are economical. On the other hand, a catalyst comprising a transition metal compound comprising halogen atoms introduced into substituents on the atoms crosslinking ligands and a co-catalyst has been proposed (as in JP-A-4-366106). However, such a catalyst is disadvantageous in that it provides a polymer having a low molecular weight and steric regularity as compared with similar catalysts free of halogen atoms.
Further, a compound has been known having enhanced isotacticity and increased molecular weight provided by adding substituents to indenyl group which is part of ligands (as in JP-A-4-268307, JP-A-6-157661). Moreover, a transition metal compound has been known wherein a subring containing two adjacent carbon atoms constituting a conjugated 5-membered ring has members other than 6 (as in JP-A-4-275294, JP-A-6-239914, JP-A-8-59724).
However, the foregoing compounds exhibit an insufficient catalytic action under high temperature polymerization conditions that are economical. Further, these compounds give a catalyst system soluble in the reaction medium in most cases. Accordingly, the resulting polymer has an amorphous grain form and a small bulk density and contains much fine powder and thus exhibits extremely poor grain properties. Accordingly, these compounds have many production disadvantages. For example, if these compounds are used in slurry polymerization or gas phase polymerization, continuous stable operation can be hardly conducted.
In order to solve these problems, on the other hand, a catalyst comprising a transition metal compound and/or organic aluminum compound supported on an inorganic oxide (e.g., silica, alumina) or organic material has been proposed (as in JP-A-61-108610, JP-A-60-135408, JP-A-61-296008, JP-A-3-74412, JP-A-3-74415). However, polymers prepared in the presence of such a catalyst contain much fine powder or coarse grains. Further, these polymers exhibit insufficient grain properties, e.g., low bulk density. Moreover, such a catalyst exhibits a low polymerization activity per unit solid component. Further, such a catalyst provides a polymer having a relatively low molecular weight and steric regularity than a catalyst system free of carrier. The present invention has been worked out under these circumstances.